The present invention relates to a display panel and a display device, in which an optical display element such as liquid crystal is driven by in plane switching in a direction along a substrate surface.
There has been a remarkable progress in the popularization of a liquid crystal display device used as a display device for a personal computer, or other various monitors. The liquid crystal display device of this kind is typically constituted in a manner that a backlight as a planar light source for illumination is provided in the backside of a liquid crystal panel, a liquid crystal surface having a predetermined area is irradiated to have an entirely uniform brightness, and thus an image formed on the liquid crystal surface of the liquid crystal panel is made visible. The liquid crystal panel is constructed in a way that a drive voltage is applied to the liquid crystal filled between two glass substrates to drive the liquid crystal. In the liquid crystal panel, the liquid crystal is driven to change an orientation of the liquid crystal (molecule) in the above-described manner. Therefore, polarization of transmitted light is controlled, and thus a desired image is displayed.
Incidentally, in the liquid crystal panel is made so that the electrodes for driving the liquid crystal have been provided on one and the other of two glass substrates, respectively, and an electric field has been generated in a direction of the two glass substrates being linked to each other, that is, in a direction approximately perpendicular to the substrate surfaces of the glass substrates, and thus the liquid crystal is driven.
However, in the liquid crystal panel of the system as described above, the viewing angle thereof has been narrow. A technology for improving the viewing angle, an in plane switching (IPS: a transverse electric field) mode is known. This is a technology, in which a drive voltage is applied to the electrodes provided in only one of the two glass substrates. An electric field in a horizontal direction (transverse direction) along the substrate surface is generated, and thus the liquid crystal is driven.
FIGS. 7 and 8 show a wiring structure of a liquid crystal panel of the conventional IPS mode. The liquid crystal panel is constructed in such a manner that a liquid crystal layer 3 filled with liquid crystal, insulating layers 4A and 4B are interpolated between glass substrates 1 and 2, and that insulating layers 4A and 4B are provided with thin film transistors (TFTs) 9 having signal lines 5, gate lines 6, common electrodes 7, pixel electrodes 8, an amorphous silicon layer and the like deposited thereon.
As shown in FIG. 8, the signal lines 5 and the gate lines 6 are arranged in a matrix type. Then, a pixel region is defined by two signal lines 5 adjacent to each other and two gate lines 6 adjacent to each other surrounding the same.
The common electrode 7 is arranged along each of the signal lines 5 located at the both sides of each pixel region. Moreover, the pixel electrode 8 is arranged in an intermediate position between two common electrodes 7, and one end of the pixel electrode 8 is connected to the TFT 9.
With regard to the liquid crystal panel thus structured, in each pixel region, a voltage is applied between the common electrode 7 and the pixel electrode 8 by an operation of the TFT 9, a transverse electric field is generated, and thus the liquid crystal is driven to perform a desired displaying.
Incidentally, in the liquid crystal panel of the conventional IPS mode as described above, there exist problems as described below.
First, the drive of the liquid crystal is originally performed by an electric field between the common electrode 7 and the pixel electrode 8. However, if an electric field is generated between the signal line 5 and the pixel electrode 8 and the liquid crystal is moved by the influence thereof, crosstalk occurs. For this reason, heretofore, by widening the width of the common electrode 7, the common electrode 7 is set to exert the shield effect so as not to extend the influence of the electric field from the signal line 5 to the pixel electrode 8. However, this results in lowering of an aperture ratio as a ratio of an aperture X to the pixel region.
Moreover, when a distance d between the signal line 5 and the common electrode 7 is small, an electric capacitance load to the signal line 5 is increased thereby not allowing the liquid crystal in the display region to be driven evenly. Therefore, the distance d is required to be enlarged. However, such enlargement of the distance d also results in lowering of the aperture ratio. Furthermore, in order to prevent crosstalk dependent on a visual angle, that is, a phenomenon that switching between the signal line 5 and the common electrode 7 is observed depending on an angle (viewing angle) relative to the liquid crystal panel (refer to an arrow in FIG. 7), heretofore, a black matrix 10 covering the signal line 5 has been required to be widened. However, such widening of the black matrix also has resulted in lowering of the aperture ratio, and in addition, has required a high accuracy in aligning the glass substrates 1 and 2.
Particularly, accompanied with higher definition of a liquid crystal panel in recent years, each pixel region has been reduced. For this reason, when improvement of the aperture ratio is hindered due to various reasons as described above, achievement of the higher definition of the liquid crystal panel is also hindered.
The inventor of the present invention has found out a method shown in FIG. 9, in which a common electrode 7′ is arranged immediately above a signal line 5. If such a construction is adopted, an electric field from the signal line 5 acts on the closest common electrode 7′ immediately above the signal line 5. Therefore, a shield effect is exerted by the common electrode 7′, and a pixel electrode 8 is hardly influenced by the electric field. However, the width of the common electrode 7′ is required to be widened as shown by a chain and dotted line in FIG. 9 in order to secure the shield effect by the common electrode 7′. Consequently, the aperture ratio is lowered. From the above, this method was not deemed as effective solving means for the problems.